who's this very old lady and why does she have 3 trash cans?

Lily's been learning about jokes lately.  For a while, she just said random stuff and then asked, "Is that a joke?" to which we'd reply, "It's interesting, but no not a joke."

I finally explained to her that a joke is a small story with something funny at the end.   So she started telling jokes that all start with the following sentence, "One upon a time there was a very old woman who had three trash cans. "

From there, she'd go on to add in other things this old woman with three trash cans had, such as three tee-shirts or a strange dog or a ball of wax.  But she always, ALWAYS had the three trash cans.  Occasionally, but not often, it'll be an old man instead of an old woman. 

And damn is it funny.  

Sadly, still not really a joke.